Asymmetric RTS/CTS for Exposed Node Reduction in IEEE 802.11 Ad Hoc Networks

Abstract

One interesting problem regarding wireless local area network (WLAN) ad-hoc networks is the effective mitigation of hidden nodes. The WLAN standard IEEE 802.11 provides request to send/clear to send (RTS/CTS) as mitigation for the hidden node problem; however, this causes the exposed node problem. The first 802.11 standard provided only two transmission rates, 1 and 2 Mbps, and control frames, such as RTS/CTS assumed to be sent at 1 Mbps. The 802.11 standard has been enhanced several times since then and now it supports multi-rate transmission up to 65 Mbps in the currently popular 802.11n (20 MHz channel, single stream with long guard interval). As a result, the difference in transmission rates and coverages between the data frame and control frame can be very large. However adjusting the RTS/CTS transmission rate to optimize network throughput has not been well investigated. In this paper, we propose a method to decrease the number of exposed nodes by increasing the RTS transmission rate to decrease RTS coverage. Our proposed method, Asymmetric Range by Multi-Rate Control (ARMRC), can decrease or even completely eliminate exposed nodes and improve the entire network throughput. Experimental results by simulation show that the network throughput in the proposed method is higher by 20% to 50% under certain conditions, and the proposed method is found to be effective in equalizing dispersion of throughput among nodes.

Optimizing RTS/CTS to Improve Throughput in Ad Hoc WLANs

“…Thus, node C is known as the exposed node in this scenario and has to hold its transmission for the Network Allocation Vector (NAV) period defined in the RTS frame. This decreases network spatial utilization and performance [6].…”

“…Our first research of the proposed method was reported in [6]. However, the research in [6] focused primarily on multi-rate transmission of RTS and CTS frames in order to control transmission range.…”

“…However, the research in [6] focused primarily on multi-rate transmission of RTS and CTS frames in order to control transmission range. Our method directly adjusts the transmission ranges of the control frames without considering transmission rate.…”

“…This is done for simplicity and to not cause complications with the PHY layer convergence procedure (PLCP) preamble whose transmission rate cannot be changed. Additionally, the simulations in [6] were limited to evaluating the basic performance of the method with regards to only next-hop neighbour node communication. Our evaluations go a step further to evaluate end-to-end network communication by taking into account the routing protocol used.…”

“…This is achieved by setting the transmission range of RTS frames to be less than that of CTS frames. We employ the fundamental method described in [6] to express the concept of asymmetric RTS/CTS transmission ranges as shown in Fig. 4.…”